Geminiviruses are plant pathogens that cause significant yield losses in crop plants in many countries of the world (Briddon et al, “Geminiviridae”, p. 158-165. In F. A. Murphy (ed.), Virus Taxonomy, Sixth Report of International Committee on Taxonomy of Viruses, Springer-Verlag, Vienna & New York, 1995; Frischmuth et al, Semin. Virol., 4:329-337, 1993; Harrison et al, Ann. Rev. Phytopathol., 23:55-82, 1985; Polston et al, Plant Dis., 81:1358-1369, 1997). Different members are transmitted by whiteflies or leafhoppers (Davies et al, Genet., 5:77-81, 1989; Lazarowitz et al, Crit. Rev. Plant Sci., 11:327-349, 1992). Most of the whitefly-transmitted geminiviruses (WTGs) have bipartite genomes while all the leafhopper-transmitted geminiviruses and some of the WTGs have monopartite genomes. The monopartite genomes (2566-3028 nt) encode proteins required for replication, encapsidation and movement, while in the case of the bipartite viruses the movement functions are encoded by a second genome component of similar size (Davies et al, Genet., 5:77-81, 1989; Ingham et al, Virology, 207:191-204, 1995; Timmermans et al, Annu. Rev. Plant Physiol. Plant Mol. Biol., 45:79-112, 1994).
Geminiviruses have circular single-stranded (ss) DNA genomes encapsidated in double icosahedral particles. Geminiviruses replicate via a rolling circle mechanism analogous to replication of bacteriophages with ssDNA genomes. The incoming geminivirus single-stranded (ss) DNA is converted by host enzymes to double-stranded (ds) DNA which in turn serves as a template for transcription of early, replication associated genes on the complementary-sense strand. Replication initiator protein (Rep or AC1) is the only viral protein required for replication. In bipartite geminiviruses, a second protein (AC3) enhances replication. AC2, another early gene product, transactivates expression of the coat protein (CP) gene on the virion-sense strand. While the CP is not required for replication of the virus in protoplasts or plants, mutations in CP lead to dramatic decreases in accumulation of ssDNA in protoplasts or plants without affecting the accumulation of dsDNA. On the other hand, tomato golden mosaic virus CP mutations had no effect on DNA accumulation in plants, but reduced ssDNA accumulation while increasing the dsDNA accumulation in protoplasts. In plants, lack of CP results in a complete loss of infectivity of monopartite viruses but not bipartite viruses.
Coat protein may influence the ratios of ss and dsDNA levels in a passive manner by depleting the ssDNA that is available for conversion to dsDNA through encapsidation, or by modulating ssDNA synthesis, or both. No evidence is available for how CP influences ssDNA accumulation in geminiviruses. In tomato leaf curl virus from New Delhi (ToLCV-NbE, hereafter referred as ToLCV), a geminivirus with bipartite genome, disrupting the synthesis of wild type CP resulted in drastic reduction in ssDNA and a three to five fold increase in dsDNA accumulation in infected protoplasts. Inoculated plants, however, develop severe symptoms and accumulate wild type levels of dsDNA and low levels of ssDNA.
There remains a need to better understand the role of CP in geminivirus replication.